1. Field of the Invention
The present invention relates to precesses for producing synthesis gas from light hydrocarbons (e.g., natural gas) and oxygen. More particularly, the invention relates to supported nickel-rhodium based catalysis, their methods of making, and to processes employing such catalysts for net partial oxidation of light hydrocarbons (e.g., natureal gas) to products comprising CO and H2.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversation of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons, for example, using the Fischer-Tropsch process to provide fuels that boil in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming, which is the most widespread process, or by dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, proceeding according to Equation 1.
CH4+H2OCO+3H2xe2x80x83xe2x80x83(1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue.
The catalytic partial oxidation of hydrocarbons, e.g., natural gas or methane to syngas is also a process known in the art. While currently limited as an industrial process, partial oxidation has recently attracted much attention due to significant inherent advantages, such as the fact that significant heat is released during the process, in contrast to steam reforming processes.
In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.
CH4+1/202 CO+2H2xe2x80x83xe2x80x83(2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol and to fuels. The partial oxidation is also exothermic, while the steam reforming reaction is strongly endothermic. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes. The syngas in turn may be converted to hydrocarbon products, for example, fuels boiling in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes by processes such as the Fischer-Tropsch Synthesis.
The selectivities of catalytic partial oxidation to the desired products, carbon monoxide and hydrogen, are controlled by several factors, but one of the most important of these factors is the choice of catalyst composition. Difficulties have arisen in the prior art in making such a choice economical. Typically, catalyst compositions have included precious metals and/or rare earths. The large volumes of expensive catalysts needed by prior art catalytic partial oxidation processes have placed these processes generally outside the limits of economic justification.
For successful operation at commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the methane feedstock at high gas hourly space velocities, and the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Such high conversion and selectivity must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (xe2x80x9ccokexe2x80x9d) on the catalyst, which severely reduces catalyst performance. Accordingly, substantial effort has been devoted in the art to the development of catalysts allowing commercial performance without coke formation.
A number of process regimes have been proposed in the art for the production of syngas via catalyzed partial oxidation reactions. For example, the process described in U.S. Pat. No. 4,877,550 employs a syngas generation process using a fluidized reaction zone. Such a process however, requires downstream separation equipment to recover entrained supported-nickel catalyst particles.
To overcome the relatively high pressure drop associated with gas flow through a fixed bed of catalyst particles, which can prevent operation at the high gas space velocities required, various structures for supporting the active catalyst in the reaction zone have been proposed. U.S. Pat. No. 5,510,056 discloses a monolithic support such as a ceramic foam or fixed catalyst bed having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. Catalysts used in that process include ruthenium, rhodium, palladium, osmium, iridium, and platinum. Data are presented for a ceramic foam supported rhodium catalyst at a rhodium loading of from 0.5-5.0 wt %.
U.S. Pat. No. 5,648,582 discloses a process for the catalytic partial oxidation of methane at space velocities of 800,000 hrxe2x88x921 to 12,000,000 hrxe2x88x921 on certain supported Rh, Ni or Pt catalysts. The exemplified catalysts are rhodium and platinum, at a loading of about 10 wt %, on alumina foams. The small catalyst bed used in this process is said to eliminate hot spots which are typical of relatively thick catalyst beds.
Catalysts containing Group VIII metals such as nickel or rhodium on a variety of supports are known in the art. For example, V. R. Choudhary et al. (xe2x80x9cOxidative Conversion of Methane to Syngas over Nickel Supported on Low Surface Area Catalyst Porous Carriers Precoated with Alkaline and Rare Earth Oxides,xe2x80x9d J. Catalysis 172:281-293 (1997)) disclose the partial oxidation of methane to syngas at contact times of 4.8 ms (at STP) over supported nickel catalysts at 700 and 800xc2x0 C. The catalysts were prepared by depositing NiOxe2x80x94MgO on different commercial low surface area porous catalyst carriers consisting of refractory compounds such as SiO2, Al2O3, SiC, ZrO2 and HfO2. The catalysts were also prepared by depositing NiO on the catalyst carriers with different alkaline and rare earth oxides such as MgO, CaO, SrO, BaO, Sm2O3 and Yb2O3. 
U.S. Pat. No. 4,690,777 also discloses catalysts comprising Group VIII metals, such as Ni, on porous supports, for use in reforming hydrocarbons to produce CO and H2. 
U.S. Pat. No. 5,149,464 discloses a method for selectively converting methane to syngas at 650xc2x0 C. to 950xc2x0 C. by contacting the methane/oxygen mixture with a solid catalyst comprising a supported d-Block transition metal, transition metal oxide, or a compound of the formula MxMxe2x80x2yOz wherein Mxe2x80x2 is a d-Block transition metal and M is Mg, B, Al, GA, Si, Ti, Xr, Hf or a lanthanide.
U.S. Pat. No. 5,500,149 discloses various transition metals that can act as catalysts in the reaction CO2+CH4xe2x86x922CO+2H2, and demonstrates how reaction conditions can affect the product yield. The partial oxidation of methane to synthesis gas using various transition metal catalysts under a range of conditions has been described by Vernon, D. F. et al. (Catalysis Letters 6:181-186 (1990)). European Pat. App. Pub. No. 640561 discloses a catalyst for the catalytic partial oxidation of hydrocarbons comprising a Group VIII metal on a refractory oxide having at least two cations.
U.S. Pat. No. 5,447,705 discloses a catalyst having a perovskite crystalline structure and the general composition: LnxA1xe2x88x92yByO3, wherein Ln is a lanthanide and A and B are different metals chosen from Group IVb, Vb, VIb, VIIb or VIII of the Periodic Table of the Elements.
U.S. Pat. No. 5,653,774 discloses a nickel-containing catalyst for preparing synthesis gas which are prepared by heating hydrotalcite-like compositions having the general formula:
[M2+(1xe2x88x92x)Mx3+(OH2)]x+(Ax/nnxe2x88x921).mH2O.
A previous attempt at synthesis gas production by catalytic partial oxidation to overcome some of the disadvantages and costs typical of steam reforming is described in European Patent No. EPO 0 303 438, entitled xe2x80x9cProduction of Methanol from Hydrocarbonaceous Feedstock.xe2x80x9d
One disadvantage of many of the existing catalytic hydrocarbon conversion methods is the need to include steam in the feed mixture to suppress coke formation on the catalyst. Another drawback of some of the existing processes is that the catalysts that are employed often result in the production of significant quantities of carbon dioxide, steam, and C2+ hydrocarbons. Also, large volumes of expensive catalyst are typically required in order to achieve satisfactory conversion of the feed gas, and to achieve the necessary level of selectivity for CO and H2 products. None of the existing processes or catalysts are capable of providing high conversion of reactant gas and high selectivity of CO and H2 reaction products. Accordingly, there is a continuing need for a process and catalyst for the catalytic partial oxidation of hydrocarbons, particularly methane, or methane containing feeds, in which the catalyst retains a higher level of activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity, elevated pressure and high temperature.
The present invention provides catalysts and processes for preparing synthesis gas from any gaseous hydrocarbon having a low boiling point (e.g. C1-C5 hydrocarbons, particularly methane, or methane containing feeds) and a source of molecular oxygen. One advantage of the new catalysts is that they retain a high level of activity and selectivity to carbon monoxide and hydrogen under conditions of high gas space velocity, elevated pressure and high temperature.
The new processes of the invention are particularly useful for converting gas from naturally occurring reserves of methane which contain carbon dioxide. Another advantage of the new catalysts and processes is that they are economically feasible for use in commercial-scale conditions. In a short contact time reactor, the new Nixe2x80x94Rh based catalysts are highly active for catalyzing the oxidation of methane to syngas by an overall or net partial oxidation process. xe2x80x9cNet partial oxidationxe2x80x9d means that the partial oxidation reaction predominates over reforming reactions, and the ratio of the H2:CO products is about 2. The short contact time reactor permits the reactant gas mixture to contact or reside on the catalyst for no longer than about 10 milliseconds, operating at high space velocities.
In accordance with certain embodiments of the present invention a supported catalyst is provided for catalyzing the net partial oxidation of a hydrocarbon. The supported catalyst contains about 1-50 weight percent nickel and about 0.01-10 weight percent rhodium, and a support structure. Preferably, the support structure comprises a spinel, a perovskite, magnesium oxide, a pyrochlore, a brownmillerite, zirconium phosphate, magnesium stabilized zirconia, zirconia stabilized alumina, silicon carbide, yttrium stabilized zirconia, calcium stabilized zirconia, yttrium aluminum garnet, alumina, cordierite, ZrO2, MgAl2O4, SiO2 or TiO2. The rhodium and nickel are disposed on or within the support structure. In some of the more preferred embodiments, the catalyst support structure is a spinel, a perovskite, a pyrochlore or a brownmillerite, and the nickel-rhodium alloy is incorporated into the support structure. In some embodiments the catalyst comprises a solid solution of rhodium-nickel-magnesium-oxide.
In certain embodiments of the catalysts the support structure comprises a refractory oxide, which may be in the form of a foam structure. Preferably such a foam structure comprises about 12-60 pores per centimeter of structure. Alternatively, the support structure may be in the form of a honeycomb monolith structure.
In some embodiments, the catalyst comprises a mixture of rhodium and nickel on a support structure comprising Al2O3. Some embodiments comprise an alloy of 1% rhodium, 3% manganese and 13% nickel on a MgAl2O4 support structure. In some embodiments the catalyst comprises 1% rhodium and LaZr0.5Ni0.5O3 (expressed as stoichiometric amounts). Another catalyst embodiment comprises Rh0.02Ni0.03Mg0.95O, and yet another comprises rhodium, nickel and cobalt on a support structure comprising ZrO2. Certain catalysts of the invention comprise 1% rhodium, 10.9% nickel and 8.6% magnesium on a support structure comprising 99% Al2O3.
Also provided by the present invention are processes for the net partial oxidization of a 1-5 carbon containing hydrocarbon to form a product gas mixture comprising CO and H2. In some embodiments the process comprises contacting a reactant gas mixture comprising the hydrocarbon and a source of oxygen with a catalytically effective amount of a Nixe2x80x94Rh alloy-containing catalyst having a composition as described above. The process also includes maintaining the catalyst and the reactant gas mixture at conversion promoting conditions of temperature, gas flow rate and feed composition during contact with the reactant gas mixture. Preferably the process includes maintaining the reactant gas mixture and the catalyst at a temperature of about 600-1,200xc2x0 C. during contact. In some embodiments the temperature is maintained at about 700-1,100xc2x0 C.
In some embodiments of the process the reactant gas mixture and the catalyst are maintained at a pressure of about 100-12,500 kPa during the contacting, and in some of the more preferred embodiments the pressure is maintained at about 130-10,000 kPa.
Certain embodiments of the processes for converting hydrocarbons to CO and H2 comprise mixing a methane-containing feedstock and an oxygen-containing feedstock to provide a reactant gas mixture feedstock having a carbon:oxygen ratio of about 1.25:1 to about 3.3:1. In some of these embodiments, the mixing step is such that it yields a reactant gas mixture feed having a carbon:oxygen ratio of about 1.3:1 to about 2.2:1, or about 1.5:1 to about 2.2:1. In some of the most preferred embodiments the mixing step provides a reactant gas mixture feed having a carbon:oxygen ratio of about 2:1.
In some embodiments of the processes the said oxygen-containing gas that is mixed with the hydrocarbon comprises steam or CO2, or a mixture of both. In some embodiments of the processes the C1-C5 hydrocarbon comprises at least about 50% methane by volume, and in some of the preferred embodiments the C1-C5 hydrocarbon comprises at least about 80% methane by volume.
Certain embodiments of the processes for preparing synthesis gas comprise preheating the reactant gas mixture. Some embodiments of the processes comprise passing the reactant gas mixture over the catalyst at a space velocity of about 20,000 to about 100,000,000 normal liters of gas per kilogram of catalyst per hour (NL/kg/h). In certain of these embodiments, the gas mixture is passed over the catalyst at a space velocity of about 50,000 to about 50,000,000 NL/kg/h. Preferably the residence time of the reactant gas mixture on the catalyst is no more than about 10 milliseconds duration.
In some embodiments of the processes of the present invention the catalyst is retained in a fixed bed reaction zone. These and other embodiments, features and advantages of the present invention will become apparent with reference to the following description.
Preferred Nixe2x80x94Rh based catalysts for catalytically converting C1-C5 hydrocarbons to CO and H2 comprise an alloy of about 1 wt % to about 50 wt % nickel and 0.01 to 10 wt % rhodium on supports of spinels, perovskites, magnesium oxide, pyrochlores, brownmillerites, zirconium phosphate, magnesium stabilized zirconia, zirconia stabilized alumina, silicon carbide, yttrium stabilized zirconia, calcium stabilized zirconia, yttrium aluminum garnet, alumina, cordierite, ZrO2, MgAl2O4, SiO2 or TiO2. The Rh and/or Ni may be incorporated into the structure of the spinel, perovskite, pyrochlore or brownmillerite. Alternatively, a solid solution of Rhxe2x80x94Nixe2x80x94Mg-oxide may be used. Preferably the catalyst is a Nixe2x80x94Rh alloy. A Rh-Ni based catalyst is prepared as described in the following examples and utilizing techniques known to those skilled in the art, such as impregnation, wash coating, adsorption, ion exchange, precipitation, co-precipitation, deposition precipitation, sol-gel method, slurry dip-coating, microwave heating, and the like, or any of the other methods known in the art. Preferred techniques are impregnation, sol-gel methods and co-precipitation. For example, a Rh-Ni based catalyst is prepared by impregnation of a ceramic foam of a refractory oxide with rhodium and nickel.
Alternatively, the catalyst components, with or without a ceramic support composition, may be extruded to prepare a three-dimensional form or structure such as a honeycomb, foam, or other suitable tortuous-path structure. Additionally the catalyst components may be added to the powdered ceramic composition and then extruded to prepare the foam or honeycomb. Suitable foams for use in the preparation of the catalyst include those having from 30 to 150 pores per inch (12 to 60 pores per centimeter). Alternative forms for the catalyst include refractory oxide honeycomb monolith structures, or other configurations having longitudinal channels or passageways permitting high space velocities with a minimal pressure drop. Such configurations are known in the art and described, for example, in Structured Catalysts and Reactors, A. Cybulski and J. A. Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599-615 (Ch. 21, X. Xu and J. A. Moulijn, xe2x80x9cTransformation of a Structured Carrier into Structured Catalystxe2x80x9d)
Any suitable reaction regime may be applied in order to contact the reactants with the catalyst. One suitable regime is a fixed bed reaction regime, in which the catalyst is retained within a reaction zone in a fixed arrangement. Particles of catalyst may be employed in the fixed bed regime, retained using fixed bed reaction techniques well known in the art. Alternatively, the catalyst may be in the form of a foam, or porous monolith.
Process of Producing Syngas
A feed stream comprising a hydrocarbon feedstock and an oxygen-containing gas is contacted with one of the above-described Rhxe2x80x94Ni alloy catalysts in a reaction zone maintained at conversion-promoting conditions effective to produce an effluent stream comprising carbon monoxide and hydrogen. Preferably a millisecond contact time reactor is employed. The hydrocarbon feedstock may be any gaseous hydrocarbon having a low boiling point, such as methane, natural gas, associated gas, or other sources of light hydrocarbons having from 1 to 5 carbon atoms. The hydrocarbon feedstock may be a gas arising from naturally occurring reserves of methane which contain carbon dioxide. Preferably, the feed comprises at least 50% by volume methane, more preferably at least 75% by volume, and most preferably at least 80% by volume methane.
The hydrocarbon feedstock is in the gaseous phase when contacting the catalyst. The hydrocarbon feedstock is contacted with the catalyst as a mixture with an oxygen-containing gas, preferably pure oxygen. The oxygen-containing gas may also comprise steam and/or CO2 in addition to oxygen. Alternatively, the hydrocarbon feedstock is contacted with the catalyst as a mixture with a gas comprising steam and/or CO2.
Preferably, the methane-containing feed and the oxygen-containing gas are mixed in such amounts to give a carbon (i.e., carbon in methane) to oxygen (i.e., oxygen) ratio from about 1.25:1 to about 3.3:1, more preferably, from about 1.3:1 to about 2.2:1, and most preferably from about 1.5:1 to about 2.2:1, especially the stoichiometric ratio of 2:1.
The process is operated at atmospheric or superatmospheric pressures, the latter being preferred. The pressures may be from about 100 kPa to about 12,500 kPa, preferably from about 130 kPa to about 10,000 kPa.
The process is preferably operated at temperatures of from about 60xc2x0 C. to about 1200xc2x0 C., preferably from about 700xc2x0 C. to about 1100xc2x0 C. The hydrocarbon feedstock and the oxygen-containing gas are preferably pre-heated before contact with the catalyst.
The hydrocarbon feedstock and the oxygen-containing gas are passed over the catalyst at any of a variety of space velocities. Space velocities for the process, stated as normal liters of gas per kilogram of catalyst per hour, are from about 20,000 to about 100,000,000 NL/kg/h, preferably from about 50,000 to about 50,000,000 NL/kg/h. Preferably the catalyst is employed in a millisecond contact time reactor for syngas production. The product gas mixture emerging from the reactor is collected and analyzed for CH4, O2, CO, H2, CO2, etc.